1. Field of the Invention
The present invention relates generally to electro-photographic image formation methods and devices such as digital copiers and laser beam printers, and particularly, to an image formation method and device which generates a pulse signal based on image data so as to form an electrostatic latent image on a photosensitive element.
2. Description of the Related Art
In a typical image formation device using an electro-photographic technique, a transferred toner image is fixed onto a sheet of transfer paper in order to form a permanent image on the paper. The toner image is usually fixed onto the transfer paper using thermal fixing. Various types of thermal-fixing techniques include hot air fixing, oven fixing, and more recently, heating roller fixing. In the heating-roller technique, a heating roller and a pressure roller are disposed in parallel to each other in the conveying path of the transfer paper, and the transfer paper is conveyed through the two rollers while being nipped therebetween. The heating roller fuses the toner, and at the same time, the pressure roller presses the fused toner against the transfer paper so as to fix the toner on the transfer paper. As an alternative to such a structure, a pressure pad or a pressure belt is provided in place of the pressure roller.
However, when using such a thermal-fixing technique to fix the toner image on the transfer paper, the heat causes the moisture in the transfer paper to evaporate and thus causes the transfer paper to shrink. The degree of shrinkage may vary depending on, for example, the material of the transfer paper or the thickness of the paper. Furthermore, it is generally known from experience that it normally takes about 15 to 20 minutes for the shrunken transfer paper to recover its original size.
To perform double-sided recording on the transfer paper, a thermal-fixing unit provided in the image formation device fixes a first toner image onto a first side of the transfer paper, and then the transfer paper is turned over so that a second toner image can be transferred to a second side of the transfer paper. Subsequently, the thermal-fixing unit fixes the second toner image on the second side of the transfer paper. On the other hand, to combine two images on a single side of the transfer paper, the thermal-fixing unit fixes a first toner image onto one side of the transfer paper, and then, without turning over the transfer paper, a second toner image is transferred to the same side of the paper. Subsequently, the thermal-fixing unit fixes the second toner image onto the paper.
In both cases, the first formed image and the second formed image are different in size because the transfer paper becomes smaller due to shrinkage after the first image is formed.
Japanese Patent Laid-Open No. 4-288560, for example, discloses a technique for solving such a problem. Specifically, an optical sensor is disposed upstream of a position where the transferring process is performed in a conveying path of the transfer paper so as to detect the longitudinal size and the lateral size of the transfer paper. Moreover, another optical sensor is disposed downstream of a thermal-fixing unit so as to similarly detect the longitudinal size and the lateral size of the transfer paper. Based on these detections, the shrinkage or expansion proportion (i.e., change or ratio of the size before thermal fixing relative to the size after thermal fixing) of the transfer paper in both the longitudinal and lateral directions is determined. Based on the determined shrinkage or expansion proportion, the scanning rate of an optical scanner is controlled.
Japanese Laid-Open No. 10-149057 discloses a technique for reducing the workload required in the above-mentioned technique. Specifically, an optical sensor is disposed upstream of a position where the transferring process is performed in the conveying path of the transfer paper so as to detect the longitudinal size of a first sheet of transfer paper before or after the fixing process is performed. Based on the detection, the shrinkage or expansion proportion in the longitudinal direction of the first sheet of transfer paper is determined. Accordingly, the scanning rate of an optical scanner is controlled for the second sheet of transfer paper onward using the shrinkage or expansion proportion of the first sheet of transfer paper.
Image forming devices also use known modulation techniques to record images based on multiple-value image data for each pixel. One modulation technique is known as Pulse Width Modulation (PWM). In PWM, the width of a pulse representing the light-emission time of a laser beam is modulated for each pixel, while maintaining the intensity of the laser beam. Another one is a technique in which the intensity of a laser beam is modulated while maintaining the light-emission time for each pixel. PWM is more commonly used since it provides a simpler control operation and more stable recording.
To prevent different sizes of formed images due to shrinkage of the transfer paper, an image formation device that uses PWM not only changes the scanning rate of the optical scanner based on the shrinkage or expansion proportion of the transfer paper, but also changes the main-scanning rate (i.e. the rotational rate of a polygon motor) of a laser beam and the sub-scanning rate (i.e. the rotational rate of a photosensitive element) of a laser beam with respect to the photosensitive element.
However, when the main-scanning rate of the laser beam is changed, the density level of each electrostatic latent image segment formed on the photosensitive element also changes. This is because the light-emission time is modulated for each pixel without changing the intensity of the emitted laser beam in the PWM technique, and therefore, when the main-scanning rate is changed, the amount of incident laser beam per unit area on the photosensitive element changes even if the recording processes are performed based on the same image data having the same density levels.
For example, when forming electrostatic latent image segments based on image data items having the same density level at a regular interval, if the main-scanning rate of the laser beam is lowered, the interval of the electrostatic latent image segments becomes small, thus causing the area of each latent image segment to become smaller. In such a case, if the pulse signal, i.e. pulse width, is not changed, the light-emission time of the laser beam, namely, the amount of incident laser beam, remains the same, meaning that the same amount of laser beam is emitted to the smaller area of each latent image segment, and therefore, the amount of incident laser beam per unit area on the entire latent image is increased. As a result, the density level of the entire electrostatic latent image is increased.